Planar reactive evaporation apparatus for the deposition of compound semiconducting films

ABSTRACT

An apparatus and method for epitaxial film formation is disclosed. Planar reactive evaporation techniques suitable for scaling are employed to produce high purity compound semiconducting films at relatively low temperatures.

RELATED APPLICATION

This application is a division of Application Ser. No. 631,981, filedNov. 14, 1975, which issued as U.S. Pat. No. 4,063,974, on Dec. 20,1977.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is directed to the vapor phase formation of compoundsemiconducting films in general and particularly to the formation ofindium phosphide and gallium phosphide films on single crystalsubstrates.

2. Description of the Prior Art

Compound semiconductor films in general have been used extensively invarious electronic devices as, for example, the fabrication of laserdiodes, microwave oscillators and photoelectric detectors. In microwaveapplications, thin film semiconductors are required for high frequencyresponse characteristics.

Semiconductor quality InP epitaxial thin films have been deposited viaChemical Vapor Deposition (CVD) (R. C. Clarke, B. D. Joyce and W. H. E.Wilgoss, Solid State Communications, Vol. 8, p. 1125, 1970) and LiquidPhase Epitaxy (LPE) (J. L. Shay, K. J. Bachmann and E. Buehler, Appl.Phys. Lett., Vol. 24, p. 192, 1974). However, these techniques requiretemperatures above 500° C. For some applications, lower substratetemperatures are required. For example, substrate decomposition limitsthe substrate temperature for certain heterojunction photoelectricdevices. Specifically, a terrestrial solar cell incorporating InPdeposited on CdS would have a potentially high energy conversionefficiency (Sigurd Wagner, J. L. Shay, K. T. Bachmann, E. Buehler, Appl.Phys. Lett., Vol. 26, p. 229, 1975), but fabrication of such a devicewould have to be accomplished below approximately 400° C. to avoid CdSdecomposition. Another application involving lower substratetemperatures would be abrupt junction microwave devices such as InP GunnEffect devices. In this instance, lower temperatures would alleviateinterdiffusion effects.

Molecular beam techniques have been used successfully by various workers(A. Y. Cho, J. of Vacuum Science and Technology, Vol. 8, p. 531, 1971;D. L. Smith and V. Y. Pickhardt, J. of Appl. Physics, Vol. 46, p. 2366,1975) for the deposition of semiconductor epitaxial films at lowtemperatures. However, problems arise with this technique in producingthe appropriate phosphorus vapor species (C. T. Foxon, B. A. Joyce, R.F. C. Farrow and R. M. Griffiths, J. Phys. D., Vol. 7, p. 2422, 1974)for InP deposition. Additionally, this technique severely limits theuniform deposition area and therefore is not scalable. This arisesbecause two sources are used. These sources are at two differenttemperatures and, therefore, must be thermally isolated from each other.This isolation requirement leads to the limited uniformity in thedeposited films (see U.S. Pat. No. 3,615,931 by J. R. Arthur et al).

Reactive evaporation techniques have been used to depositpolycrystalline semiconductor compounds on single crystal substrates.(F. J. Morris et al, J. Vac Sci. Technol., Vol. II, No. 2, page 506,March 1974.) However, films deposited via this technique were neithersingle crystal films nor of high purity and therefore were not ofsemiconductor quality. The techniques disclosed by Morris et al are notreadily scalable and fail to control sources of impurities during thedeposition process.

There are no prior art processes known to me which facilitate thesimultaneous production of high purity semiconductor quality films atlow processing temperatures by a scalable method.

THE INVENTION Summary

Objectives of this invention are: to deposit heteroepitaxiallysemiconductor quality (≦ 5 ppm impurity content) thin (≦ 5μ) films; todeposit these films on substrates at relatively low temperatures (≦ 500°C.); to deposit these films at economical rates (˜ 5μ/hour); and todeposit these films in a fashion amenable to eventual production scales.

In meeting these objectives, a novel scalable vapor phase depositionapparatus and method for the preparation of compound semiconductor (MX)films which meets all of the above-listed objectives while avoiding thepitfalls of prior art processes has been developed.

The key to this process lies in the development of a method forcontrolling the production of M and X component vapors. This method wasrendered feasible with the development of a novel source design for theproduction of M and X component vapors. This source consists of aperforated dish made from an inert material with a hollow cavity intowhich an X component hydride gas is introduced. X may be any elementfrom Group V or VI of the Periodic Table.

The M component (a metal from Group II or III of the Periodic Table) isplaced in the perforated dish and elevated to a temperature suitable forevaporation of the metal component and the dissociation of the X hydridegas into X vapors and hydrogen gas. The metal vapor pressure and the Xcomponent vapor pressure are controlled independently by the cavitytemperature and an X hydride metering value.

This source is placed in a chamber pumped by a turbomolecular pump toensure removal of unreacted hydrogen gas and other residual impurities.The source and substrate region is surrounded by a liquid nitrogencooled shroud which also aids in reducing residual impurity gaspressures. This shroud effectively forms an inner source substratedeposition chamber providing an improved impurity level depositionregion.

BRIEF DESCRIPTION OF THE DRAWINGS

The nature or scope of this invention may be readily understood byreference to the drawings where

FIG. 1 is a schematic line drawing of the Planar Reactive EvaporativeDeposition Apparatus,

FIG. 2 is an expanded plan view of the preferred Inner Source SubstrateDeposition Chamber, and

FIG. 3 is an expanded plan view of an alternative design for the MXsource chamber.

DESCRIPTION OF THE INVENTION

MX compounds, where M is a metal taken from Groups II and III of theperiodic table and X is an element taken from Groups V and VI of theperiodic table, have utility in the preparation of semiconducting films.Thin films comprised of single crystal MX compounds coated onto singlecrystal substrates are exceedingly difficult to prepare and are verycostly. This is particularly true of those MX compounds which decomposeat elevated temperatures with high X component vapor pressures. Notableexamples of such compounds are InP, GaP, and CdS. A large part of thedifficulty in preparing such films is attributed to the problemsassociated with handling the X component vapor species of the MXcompound.

In order to facilitate the handling of X component vapor species, areactive evaporative deposition technique has been devised in which theX component is introduced into the deposition environment via X hydridedecomposition. Two immediate advantages of this technique are the easeof control of the X component vapor species and source scalability. Thecontrol of X component vapor pressure allows independent adjustment ofthe X component to M component pressure ratio which facilitates theproduction of stoichiometric films. Since the source is planar, it isscalable and therefore large substrate areas may be coated. Thisfacilitates large production rates.

The ability to control the component vapor pressures allows thedeposition of MX compounds at a lower rate which favors lower depositionsubstrate temperatures. Temperatures on the order of 350° C. and lessthan 500° C. have been shown to be adequate when the deposition rate isless than 5μ/hour. Chemical Vapor Deposition techniques, in contrast,generally involve deposition rates on the order of 50μ/hour andtherefore require epitaxial temperatures about 500° C.

High purity films are obtained by operating with residual gas pressuresof less than 10⁻⁹ torr. Such low pressures are obtained by operation ina high vacuum chamber (<10⁻⁸ torr) and employing a liquid nitrogencooled shroud to create an inner source substrate deposition chamber.

Thin films are prepared by the technique described above by utilizing aplanar reactive evaporative apparatus as shown in FIG. 1. The essentialfeatures of this apparatus are: a vacuum chamber enclosure 10 fabricatedfrom metal in accordance with conventional vacuum technologies; an Xcomponent hydride inlet line 11 fabricated from stainless steel equippedwith a metering valve 12 which serves to control the X componentintroduction rate into an inner source substrate deposition chamber 13.A rotatable substrate mounting plate 14 fabricated from molybdenum ismounted above the deposition chamber 13 on plate support bearings 15,mounted on the inner walls of the vacuum chamber enclosure 10.

Rotation of the substrate plate 14 is manually accomplished via abellows sealed rotating feedthrough 16 which extends through the chamberlid 17.

Substrates to be coated are mounted on the substrate plate 14 and heatedvia substrate heater lamps 18 mounted in a heater chamber 19 attached tothe inner surface of the chamber lid 17.

The vacuum chamber 10 is connected to a turbo-molecular pump 20 used tomaintain a high vacuum and remove residual gases. A residual gasanalyzer 21 is used to monitor the impurity gas levels within thechamber 10.

A dopant source oven 22 is mounted below the inner source substratedeposition chamber 13 on an inlet feedthrough line 23 which extendsthrough the vacuum chamber base plate 24 and connects to the meteringvalve 12 which controls the flow of X component hydride gas from thehydride inlet line 11.

A more detailed drawing of the inner source substrate chamber 13 and thedopant source oven 22 is shown in FIG. 2. Here, X component hydride gasflowing through the inlet feedthrough line 23 passes through the dopantsource oven 22. The dopant source oven 22 is fabricated from molybdenumor tantalum. The temperature of the oven 22 is controlled by heatercoils 30 wrapped about its periphery and monitored by a thermocouple nowshown.

The dopant 32 in the dopant source oven 22 is vaporized and carried byan X component hydride gas stream into the source cavity 33 via a sourcecavity inlet tube 34.

The walls and the bottom surface of the source cavity 33 are composed ofultra pure alumina, graphite, or boron nitride. The source cavity 33 iselectrically insulated from a graphite heater plate 35 by non-conductinginsulator spacers 36. The graphite heater plate 35 is electricallyheated by current which flows between water cooled copper electrodes 37.Electrical coupling between the heater plate 35 and the copperelectrodes 37 is accomplished via a liquid gallium coupling 38.

The source cavity 33 is equipped with a perforated sapphire top plate 39which serves to direct M, X, and H₂ vapor streams towards the substratesto be coated.

The temperature of the M component of the MX compound 40 contained inthe bottom of the source cavity 33 is monitored by a tantalum sheathedthermocouple 41.

Refractory metal heat shields 42 surround the source cavity 33 andthermally isolate the source cavity 33 and heater plate 35 from a cooledmetallic shroud 43. The shroud 43 provides a condensation surfaceprotecting the inner deposition chamber 13 from residual gas impuritiespresent in the outer vacuum chamber 10. Further protection againstresidual gases entering the deposition chamber is obtained by anoptically dense metal baffle 44 mounted onto the top of the shroud 43.The metal baffle 44 is sized such that the distance between it and thesubstrate mounting plate 14 is minimal.

Coolants for the shroud 43 and an electrical current for the watercooled copper electrodes 37 are provided from a side port 45 located inthe vacuum chamber wall 10.

An alternative source cavity design is shown in FIG. 3. This sourcedesign differs from that of 33 in FIG. 2 in that it operates with the Mvapor in a free evaporative condition. The Knudson source design of FIG.2 operates with the M component vapor pressure in the equilibriumcondition.

The above-described apparatus is used to prepare compound semiconductingfilms in general as follows: single crystal substrates 45 to be coatedare mounted on the substrate mounting plate 14 and placed in the vacuumchamber 10 on the plate support bearings 15 mounted on the inner wall ofthe vacuum chamber. The standard sample load configuration consists ofseveral samples in each of three of the four substrate plate quadrantpositions. The fourth quadrant position contains a blank plate.Initially, this blank position is centered above the source chamber forsource bakeout. The loading and unloading of samples is done in flowingdry nitrogen or argon gas to reduce the water vapor build-up on thechamber walls.

The source and dopant chambers will have been previously loaded with asuitable M component and dopant.

The apparatus is closed and evacuated to <10⁻⁸ torr with theturbomolecular pump 20. The use of a turbo-molecular pump instead of adiffusion pump is necessary to maintain an oil-free system, and also toallow good pumping speeds even at moderate chamber pressures. Thisfollows because even an adequately trapped diffusion pump should bethrottled down at pressures above 3 × 10⁻⁴ torr to avoid oil vaporback-streaming.

The substrate heaters 18 are activated and the substrate plate 14temperature is raised to a temperature between the film epitaxialtemperature and substrate decomposition temperature. The sourcetemperature is then raised to approximately 100° C. below thetemperature at which the M component vapor pressure becomes appreciablebut above the decomposition temperature of the x component hydride. Thesource temperature and substrate plate temperature are held at thesevalues until residual gases are baked out of the inner depositionchamber and the pressure restabilizes. The pressures of the variousresidual gases are monitored with the residual gas analyzer 21.

The temperature of the shroud is lowered by the introduction of a liquidcoolant (preferably liquid nitrogen). This step provides for a reductionof residual gas impurities within the chamber 10. The substrate samples46 may now be cleansed of any oxide surface coatings by thermally bakingthem either in an ultra high vacuum or in the presence of an X componentvapor species and hydrogen gas. The cleansing method utilized will varyas a function of substrate material composition.

In the case of InP films deposited epitaxially on InP single crystals,removal of the oxide surface coating is accomplished by heating the InPsubstrates in the presence of phosphorus vapors and hydrogen gas.

In the case of InP films deposited epitaxially on CdS single crystals,removal of oxide surface coatings may be accomplished by thermallybaking the CdS substrates in a high vacuum.

Once the shroud has been cooled and the system stabilized, the meteringvalve 12 is opened to allow X component hydride gas to enter the sourcechamber 33. When the pressure within the chamber stabilizes, the sourcetemperature is increased to a value such that the M component vaporpressure reaches the desired level and deposition begins.

Dopants may be provided via one of two methods. The first method wouldinvolve placing a solid dopant source in the dopant oven 22 and raisingthe temperature of the oven to a predetermined level for the desireddopant concentration during the deposition on the blank substratequadrant. In the second case the dopant may be premixed with the Xcomponent hydride and introduced with the X component hydride when themetering valve is opened.

The deposition rate is a function of the source temperature. Sourcecleaning is continued on the blank sample position for a brief periodand then the samples are rotated into position in succession fordeposition.

Specific control parameters for the use of this apparatus and method toprepare InP films are shown below: In the source chamber, the Mcomponent is In and the X component hydride is phosphine (PH₃).Substrate temperatures of 350°-400° C. are employed. The sourcetemperature is initially raised to 700° C. for bakeout. This temperatureexceeds the decomposition temperature of PH₃. A needle valve settingsufficient to yield a hydrogen plus phosphorus vapor pressure within thechamber of 3 × 10⁻⁴ torr and a source temperature of 900° C. give adeposition rate of 2μ/hr.

Extension to materials other than InP is accomplished as follows: astraightforward substitution of Ga for In would allow GaP deposition,and a further substitution of AsH₃ gas for PH₃ gas would allow thedeposition of InAs and GaAs. Similarly, substituting CdS for In and H₂ Sfor PH₃ would allow CdS deposition and likewise for other II-IV or IV-VImaterials. The method is most useful for the deposition ofstoichiometric compound semiconductor films in which one of the speciesin the compound is quite volatile.

Although in the above description pure X component hydride gases wereintroduced through the metering valve, it is also possible to usemixtures of X component hydride gas and hydrogen gas. In this instance,the extra hydrogen gas would act as a carrier gas and provide a reducingatmosphere in the deposition chamber. This would allow a reduction ofoxygen impurities incorporated in the growing films.

What is claimed is:
 1. A planar source cavity for use in a vacuumdeposition chamber at temperatures in excess of 300° C. where pressuresless than 10⁻⁸ torr are created to facilitate the preparation ofheteroepitaxial compound semiconductor films onto single crystalsubstrates comprising an inert cylindrically shaped dish mounted ontonon-conducting spacers joining said dish to an electrical heating meansand providing electrical insulation from said heating means, an inletline extending through the bottom of said dish to a means forcontrolling the flow of gaseous material into said dish, and aperforated top mounted on the top of said dish thereby forming anenclosed cavity which facilitates the control of gaseous vapor pressurescreated within said dish and directs the flow of said gaseous vaporsfrom said dish in a direction perpendicular to said top when said sourcecavity is enclosed in said deposition chamber.
 2. A planar source cavityof claim 1 wherein said top is comprised of a multiplicity ofnon-perforated recesses interdispersed between said perforations wherebyM component vapors are produced in a free evaporative condition.